/Engine/Source/Runtime/PakFile/Public/IPlatformFilePak.h
0x00000000 - 0x7ffffffe: EncodedOffset from 0 to MaxIndex 0x7fffffff: Unused, interpreted as Invalid 0x80000000: Invalid 0x80000001 - 0xffffffff: FileIndex from MaxIndex to 0
static const int32 Invalid = ((int32) 0x80000000);